As known, impact resistant vinyl aromatic copolymers are obtained by bulk or bulk-suspension polymeryzation of olefinic rubber solutions, such as ethylenepropylene (EPM) or ethylene-propylene-non conjugated diene (EPDM) solutions, in a vinyl aromatic monomer alone or in blend with other polymerizable ethylenically unsaturated monomers.
Styrene-acrylonitrile copolymers grafted on olefinic rubbers (AES) show a very good impact resistance but their low thermoresistance makes them unsuitable for particular applications where temperatures of 120.degree.-160.degree. C. are used, or for manufacturing products which come into contact with parts heated at such temperatures.
It is known that a way of improving those missing property of AES copolymers is to provide a blend with other polymers having a good thermoresistance such as for example a polyamide. In fact, such resin shows a good elongation, high energy absorption, especially in ball drop tests, and a very good thermoresistance; however, the high notch sensitivity and water absorbtion constitute a significant limitation for the use of these resins. Moreover, polyamides are seldom compatible with the impact resistant vinyl aromatic copolymers, therefore, the interfaces between the domains of the blend components represent very weak areas along which the breaking of the material takes place.
In order to make polyamides compatible with impact resistant vinyl aromatic copolymers, it was proposed in the published German patent Application N. 3, 120, 803 to use a compatibilizer constituted by a styrene copolymer containing from 2 to 50% by weight of carboxylic acid and/or reactive anhydride groups. The compatibilizing copolymer described in that reference has a molecular weight between 500 and 20.000; in particular 500 and 7.000, preferably between 800 and 2.000.
The so obtained mixtures have improved impact strength (IZOD) properties which, however, are not completely satisfactory for several applications.
According to the published European patent Application No. 202214, the impact strength of the polyamide compositions can be improved by preparing a blend comprising:
a) 5-79.5% by weight of a composition grafted on rubber consisting of graft copolymer of 15-85% by weight of at least one monomer selected from the group consisting of C.sub.1 -C.sub.4 alkyl methacrylate, C.sub.1 -C.sub.4 alkyl acrylate, metha-acrylonitrile and acrylonitrile and of 85-15% by weight of a vinyl aromatic monomer, wherein the monomers are polymerized in the presence of and grafted on a rubber substrate having a glass transition temperature below 0.degree. C., wherein the percentage by weight of rubber is from 5 to 80% and the percentage by weight of graft copolymer is from 95 to 20%;
b) 94.5-20% by weight of a polyamide, and
c) 0.5-60% by weight of a compatibilizing copolymer comprising a copolymerized, functionalized monomer able to react with the polyamide, wherein the functionalized monomer concentration is comprised between 0.05 and 4% mol., with respect to the copolymer.
However, the so obtained compositions, do not have an optimal combination of properties for all of the applications in they can be used.
In particular, an increase in impact strength can be obtained by using an excess of impact resistant vinyl aromatic polymer with respect to the polyamide resin; however, this increase is at the cost of the thermoresistance. Therefore the obtained products can't be processed at a temperature over 120.degree.-140.degree. C. On the contrary, by increasing the polyamide resin content, the thermoresistance of the composition increases, however, it is achieved at the cost of the impact strength.